|  | 5.1 | The Network Layer Basics
| 5.1.5 | Router forwarding decisions
After examining a packet's destination protocol address, the router determines that it either knows or does not know how to forward the packet to the next hop. If the router does not know how to forward the packet and there is no default route, it typically drops the packet. If the router knows how to forward the packet, it changes the destination physical address to that of the next hop and transmits the packet. The next hop may or may not be directly connected to the ultimate destination host. If it is not directly connected to the ultimate destination, the next hop is usually another router, which executes the same routing decision process as the previous router. The network address consists of a network portion and a host portion; the network portion is used by the router within the network cloud. To see whether the destination is on the same physical network, the network portion of the destination IP address is extracted and compared with the source's network address. When a packet traverses a network, the source and destination IP addresses are never changed. An IP address is computed by the IP routing protocol and software, and is known as the next-hop address. The network portion of the address is used to make path selections. A router is responsible for passing the packet to the next network along the path. The switching function allows a router to accept a packet on one interface and forward it on a second interface. The path determination function enables the router to select the most appropriate interface for forwarding a packet.
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